<!DOCTYPE html>
<html>
  <head>
    <meta charset="utf-8">
    <title>rgba,has和hsla,a代表透明度</title>
    <style media="screen">
      div{
        width: 800px;
        border: 1px solid rgb(37, 181, 80);
      }
      .div1{
        height: 500px;
        background: url('sky.jpg') top left no-repeat;
      }
      .div-child{
        height: 200px;
        background-color: rgba(0, 0,255,0.5);
      }
    </style>
  </head>
  <body>
    <div class="div1">
      <div class="div-child">
        <img src="sky.jpg" alt="天空">
      </div>
    </div>
  </body>
</html>
